1. Field of the Invention
The present invention relates to pumped solid-state lasers, amplifiers and methods for using same and, particularly, when such pumping is provided by one or more laser diodes. The invention can be used in high power diode pumped lasers for applications such as materials processing. The invention can also be used in low power diode pumped lasers for applications such as marking, cutting, drilling, machining, and communications. The invention can also be used in amplifiers for amplifying laser beams. The invention offers particular advantages for amplifying pulsed laser beams such as those produced by q-switched and/or mode locked lasers.
2. Background Art
Most solid state laser applications benefit from the use of laser sources which have high beam quality, high efficiency, and high reliability, and which are low in cost. When compared to lamp-pumped, solid state lasers, LPL""s, diode-pumped, solid state lasers, DPL""s, offer significant advantages in terms of beam quality, efficiency, and reliability, but their cost effectiveness is hampered by the high cost of laser diodes.
Typically, the pump diodes are the single most expensive component in a diode-pumped solid-state laser. The diode cost may be minimized by utilizing a DPL design with a high optical-to-optical conversion efficiency (the percentage of output power from the pump diodes which is converted to useful output from the DPL). For a higher optical-to-optical efficiency, a lower diode pump power is required to achieve a given output power. High optical-to-optical efficiency also benefits the overall system efficiency and helps improve system reliability. Using diodes without beam conditioning optics (microlenses or fiber coupling) also helps reduce the cost of the pump diodes. Diodes with integrated microlenses or fiber coupling are significantly more expensive and are lower in efficiency because 10% to 20% of the diode output is normally lost in the beam conditioning optics. Additionally, utilizing diodes with simple packages can minimize the cost of the pump diodes. Typically, if the diodes must be packed very close together, expensive micro-channel heatsinks must be used. DPL designs that use diode bars individually mounted or stacked with a wide bar-to-bar spacing may benefit from the ability to use lower cost diode packages. In order to minimize the cost of the pump diodes, the ideal DPL design should have a high optical-to-optical conversion efficiency, should not require beam conditioning optics for the pump diodes, and should permit the use of diodes with simple packaging.
A variety of laser crystals and glasses may be used as the gain medium for DPL""s. The most commonly used crystal for high power DPL""s is Neodymium doped Yttrium Aluminum Garnet, Nd:YAG. YAG is a synthetic crystal with good thermal, mechanical, and optical properties. When doped with about 1% atomic Nd, it exhibits a number of strong four-level lasing transitions. The strongest line is at 1064 nm. Commercially available laser diodes at 808 nm and 880 nm are typically used to pump Nd:YAG.
For most types of lasers, and, in particular, for solid-state lasers, thermal effects in the gain medium hamper achieving high beam quality during high output power operation. In solid-state lasers, the gain medium is normally pumped throughout its volume and cooled on one or more surfaces. This volume heating and surface cooling leads to thermal gradients in the gain medium. These thermal gradients cause stress gradients in the gain medium because thermal expansion in the hotter part of the gain medium is constrained by the cooler part of the gain medium. Because the refractive index of the gain medium is dependent on both temperature and stress, the thermal and stress gradients in the gain medium create refractive index gradients. Light traveling in the gain medium perpendicular to these gradients will experience focusing effects because the refractive index gradient makes the gain medium act as a gradient index lens. Achieving high output power and high beam quality simultaneously requires taking some steps to minimize the impact of these effects on the laser performance.
Many different DPL designs have been developed in the effort to achieve high power, high beam quality, high efficiency, high reliability, and low cost. The most common configuration is the rod-geometry DPL. In a rod-geometry DPL, the gain medium is shaped as a cylinder. It is pumped either through its side surface or through its end surface(s) and is cooled on its side surface. The beam propagates along the axis of the rod.
In rod-geometry solid-state lasers the thermal gradients are radial and light traveling down the length of the rod is focused. The strength of this xe2x80x9cthermal lensingxe2x80x9d is directly proportional to the pumping power. This thermal lensing limits the beam quality of high power, rod-geometry solid-state lasers making them a poor choice for high power, high beam quality applications. Rod-geometry DPL""s are relatively simple to build, can be designed using diodes without beam conditioning optics and have reasonable efficiencies. Rod-geometry DPL""s are currently available at kilowatt average power levels. An exemplary rod-geometry DPL is generally indicated at 10 in FIG. 1. The DPL 10 includes a laser diode stack 12 and lenses 14 which focus pump beams 16 through apertures formed in a tube 17. The focused light travels through cooling water in a flow tube 18 and into a YAG rod 20.
Numerous alternative solid-state laser geometries have been developed which use gain media with different shapes, beam paths, pumping arrangements, and cooling techniques in order to achieve high power operation at a high beam quality level. These designs include zigzag slab lasers, thin disk lasers, and planar waveguide lasers. Each of these designs utilizes cooling of a flat surface on the gain medium to produce a thermal gradient that is one-dimensional.
Zigzag slab lasers use a gain medium that is rectangular in cross section transverse to the beam propagation direction. The longer of the two opposing surfaces of the rectangle is cooled while the adjacent faces are uncooled. This establishes a one dimension thermal gradient perpendicular to the two cooled faces. Pumping can be either through the cooled faces or the uncooled faces. The beam path through the active medium follows a zigzag path making multiple reflections off the two cooled faces. The zigzag path has the effect of averaging the thermal gradient seen by any part of the laser beam such that thermal lensing is eliminated to first order. Second order effects still tend to hamper the beam quality at high power. The beam quality is typically different in the zigzag direction and the transverse direction. DPL""s of this type typically require pump laser diodes 44 to be packed close together in order to minimize the required length of the gain medium. The precision required in the fabrication of the slab itself makes it significantly more expensive than a rod of comparable length. Several companies offer high power zigzag slab DPL""s with power levels as high as 3 kW. TEMoo output powers from zigzag slab DPL""s have been limited to about 100W. A diagram of this design is shown in FIG. 2 wherein cooling water is indicated at 22.
Thin disk lasers use a disk-shaped piece of laser crystal that has a diameter much larger than its thickness. It is cooled on one of its large flat surfaces. The cooled surface acts as a mirror in the beam path of the laser and the beam is amplified as it passes through the disk before and after reflection from the mirrored surface. Because the beam is traveling in the same direction as the thermal gradient in the laser crystal there is, in principle, no thermal lensing. Again, second order thermal effects are the ultimate limitation to beam quality at high power.
This type of laser was originally developed for fusion research using lamp pumping of one of the large faces. More recently, a version of this laser was developed and patented by researchers in Germany (U.S. Pat. No. 5,553,088, Brauch, et al.). This version is based on mounting one of the faces of the disk to a solid heatsink and using diodes to pump the disk from the opposite face or from the edges. This design has shown TEMoo beam qualities up to 100W and has been scaled to powers as high as 650W with beam quality  less than 10 times diffraction limited (about 5-7 times higher beam quality than current lamp pumped 1 kW rod geometry lasers and about 3 times higher beam quality than a typical 1 kW diode pumped rod geometry laser). An exemplary thin disk laser is generally indicated at 24 in FIG. 3. A fiber bundle 26 is located adjacent a crystal and heatsink 28 which, in turn, is located adjacent a flat mirror 30. A planar array of spherical imaging mirrors 32 image the light from the crystal. An output coupler 34 is also provided.
Recently, the planar waveguide laser geometry has emerged as another alternative geometry for diode-pumped, solid-state lasers. Planar waveguide lasers, such as the one generally indicated at 36 in FIG. 4, have many attributes not generally found in any other diode-pumped, solid-state lasers. In a planar waveguide DPL the gain medium is a sheet 38 a few microns to a few hundred microns thick which acts as the core of a one-dimensional waveguide. This core 38 is typically attached to a substrate 40 that serves as a cladding on one side of the core 38. If the opposite side of the core 38 is clad with the same material as the substrate, the waveguide is referred to as a symmetrical waveguide. If the opposite side of the core 38 is unclad or clad with a material different from that of the substrate, the waveguide is referred to as an asymmetrical waveguide. FIG. 4 shows a cladding 42 on top of the core 38. In some cases there may be multiple claddings on one or both sides of the core 38. The refractive index of the core 38 is higher than that of the claddings. In the guided direction, this refractive index difference defines an acceptance angle or Numerical Aperture (NA) 46 into which light will be guided through the core 38 with low loss via total internal reflections at the core/clad interface. The NA of the waveguide is defined by the equation NA=sin("THgr"/2) where "THgr" is the full acceptance angle. For a symmetric waveguide, the NA may be calculated based on the refractive indices of the core and cladding according to the equation NA=sqrt(n02xe2x88x92n12) where n0 is the refractive index of the core 38 and n1, is the refractive index of the cladding. In the transverse, unguided, direction the light propagates as it would through a bulk section of the gain medium.
Planar waveguides may be side pumped, end pumped or face pumped by introducing the pump light into the sides, ends, or faces of the core, respectively. Side pumping and end pumping offer very high efficiency if the pump light is coupled into the core within the NA of the waveguide and if the width and length are long enough to provide significant absorption of the pump light. Face pumping is typically less efficient because the core is too thin to provide significant absorption of the pump light on a single pass. In this case the waveguide is typically surrounded by a reflective cavity which will redirect the transmitted pump light back into the core multiple times. Losses in the reflective cavity contribute to reduced efficiency when face pumping.
The side-pumped geometry for planar waveguide lasers lends itself to butt-coupling of the diode laser light into the core layer without the need for any radiance conditioning optics. As a result of butt-coupling, the planar waveguide laser system can be very compact, rugged, portable, simple to operate, and inexpensive. As the output power of 10 mm long laser diode bars continues to increase from 20 W to 40 W and beyond, higher laser output will become available from the same compact waveguide package due to the aggressive thermal engineering that is intrinsic to the approach.
Planar waveguides are typically face cooled by attaching a heatsink to the outer face of the substrate and/or cladding. Face cooling causes the heat to flow perpendicular to the plane of the core resulting in an essentially one-dimensional thermal gradient in the core. Thermal effects during laser operation are minimized because the lasing region is about 2 orders of magnitude thinner than that used in rod or slab lasers. Temperature differences between the center of the guide and the edge are on the order of 0.1xc2x0 C., and can be neglected. The minimal temperature gradient in the guided direction, combined with the guiding effect of the waveguide structure eliminate any thermally induced optical effects like the thermal tensing seen in rod geometry lasers. The thermal gradient within the core in the transverse direction is also minimal and compatible with high beam quality operation.
As a consequence of the essential absence of thermal effects in planar waveguide lasers, the beam quality can be close to diffraction limited (M2 less than 1.2) and the beam quality is not significantly affected by changes in pumping power. Additionally, scaling from low average power to powers of 10s or even 100s of watts does not require corrections in the laser architecture to counteract the increased waste heat generation as would be expected for rods or slabs. The length of the pumped region can readily be increased from around 1 cm for 10 W class lasers to several cm for power scaling. This relatively short length is in contrast to fiber lasers that employ fiber lengths on the order of several meters to 10s of meters. Although fiber lasers have demonstrated good power scalability, they run into limitations due to Raman scattering or other nonlinear interactions between the developed laser radiation and the laser gain medium if short pulses are desired. Planar waveguide lasers have no such limitations due to their very short gain medium lengths. Additionally, because cavity lengths on the order of a few cm characterize the laser resonators for waveguide lasers, nanosecond and sub-nanosecond q-switched output pulses can be readily generated.
Planar waveguides can be designed to produce single mode beam quality in the guided direction if the thickness of the core is below the cutoff thickness for propagation of the next higher order mode. This cutoff thickness is related to the NA of the waveguide. For typical NA""s, the maximum core thickness for single mode operation can range from a few microns to a few tens of microns. For a larger core thickness, the beam quality will be multimode in the guided direction unless some additional mode control technique is used.
In the transverse direction, the beam quality of a planar waveguide laser is determined by the type of resonator used. The resonator can be either stable or unstable. The resonator mirrors can be fabricated directly on the ends of the waveguide or they can be external optics. An unstable resonator fabricated directly on the ends of the waveguide offers high beam quality operation from a simple, robust, monolithic device. By incorporating appropriate devices or features into the resonator, planar waveguide lasers may be polarized, q-switched, mode-locked, or frequency shifted.
The core in a planar waveguide has the same laser characteristics as the bulk medium used. High gains can be generated in planar waveguides because pump light is confined to a small guiding region. This high pumping density in planar waveguides offers features that are not matched by other solid-state laser architectures. Besides being operational for the three common Nd3+ lasing transitions, planar waveguides are ideally suited for efficient and power-scalable operation of quasi-3-level lasing ions, such as Yb3+, Tm3+, and Er3+. Losses are typically less than 0.2 dB/cm and can potentially be reduced farther through improvements in the fabrication process. The low loss combined with the high gain in a planar waveguide permit the operation of additional laser transitions having a low gain, as is the case for tunable lasers based on Cr3+ in a number of lasing media.
Data regarding diode pumped planar waveguide lasers has been published by ORC Southampton, Heriot Watt University, and Maxios Laser Corporation among others. Maxios has obtained a patent which describes its planar waveguide laser design, U.S. Pat. No. 6,160,824, which is incorporated here in its entirety.
The Maxios laser, generally indicated at 48 in FIG. 5, uses a double-clad structure as shown to contain the pump light and to provide high beam quality output and also utilizes stress-induced birefringence to control the polarization. The NA of the interface between a core 50 and an inner clad 52 is low, about 0.02, to provide mode control but the NA of the interface between the inner clad 52 and an outer clad 54 is high, about 0.5, to provide pump confinement. The Maxios design achieves high beam quality and is reasonably simple and efficient. The structure used by Maxios is fabricated by diffusion bonding bulk material and polishing it down to the required thickness. The biggest drawback this type of double-clad design is that the waveguide 48 is a five-layer structure with three layers 50 and 52 that are very thin, 5-50 microns thick. The complex structure makes fabrication difficult. An additional drawback is that the absorption rate of the pump light from a pump diode 56 in the core 50 is reduced by a factor equal to the ratio of the core thickness to the total thickness of the core 50 and inner cladding layers 52. This is because the pump light fills both the core 50 and the inner cladding layers 52 but there is no absorption of the pump light while it is bounding through the inner cladding layers 52. A wider waveguide must be used to compensate for the lower absorption rate. To minimize the absorption rate reduction caused by the inner clad layers 52, the inner clad layers 52 are kept extremely thin (less thanxc2xd the thickness of the core 50), contributing to the fabrication difficulties.
The U.S. patent to Fermann et al., U.S. Pat. No. 5,818,630, discloses single-mode amplifiers and compressors based on multi-mode fibers. Multi-mode fibers amplify laser light in a single-mode amplifier system.
An object of the present invention is to provide an improved waveguide device with mode control and pump light confinement and method of using same.
Another object of the present invention is to provide a waveguide device with mode control and pump light confinement and method of using same wherein the device can deliver desired power, beam quality, efficiency, and reliability while utilizing a minimum amount of pump power such as diode pump power.
In carrying out the above objects and other objects of the present invention, a waveguide device which acts as a waveguide in at least one direction thereof is provided. The device includes a core having a pump input surface for receiving pumping radiation at a pumping wavelength and at least one output surface for emitting a laser beam at an output wavelength, and means for providing pump-light confinement and means for providing output mode control in different sections of the device along the direction of beam propagation.
The guided direction is preferably not the same as the direction of beam propagation. The direction of beam propagation, also called the optical axis, runs along the length of the waveguide. The guided direction in the planar waveguides is perpendicular to the plane of the core.
The core may be a single member and may include an active core member and a passive core member.
The means for providing pump-light confinement may include a pump-light containment component which may be in contact with a surface of the core in a pumping section of the device, and wherein the pump-light containment component may be a pump cladding.
The means for providing output mode control may include a coating in contact with the core, a mode control cladding in contact with the core, or a grating in contact with the core.
The core may be a planar core or may be a cylindrical core.
The device may further include a substrate for supporting the core.
The device may be a laser and wherein the laser may be a planar waveguide laser.
The core may have a laser input surface for receiving a source laser beam to be amplified and wherein the device is a optical amplifier.
The core may be planar and wherein the optical amplifier is a planar waveguide amplifier.
The laser input surface may be different from either the pumping input surface or the at least one output surface.
The laser input surface may be the same as the at least one output surface.
An output mode control section of the device may have a lower NA than the pumping section of the device.
The pumping section may have a NA greater than 0.05.
The output mode control section may have a NA less than 0.22.
The planar core may include doped YAG.
The pump cladding may have a lower refractive index than the refractive index of the core.
The pump cladding may be sapphire or undoped YAG.
The mode control cladding may include a material having a refractive index between that of the core and that of the pump cladding.
The mode control cladding may include doped or undoped YAG.
The planar core may include a first core member which absorbs the pumping radiation and a separate second core member which either does not absorb the pumping radiation or has an absorption lower than absorption of the first core member at the pumping wavelength.
The device may act as a pair of separate waveguides which are butt-coupled or coupled together by an imaging system.
The device may be an optical fiber.
The means for providing output mode control may include a mode control cladding different from the pump cladding.
The device may comprise sections of different types of fiber which are either spliced, butt-coupled or coupled together by imaging an output from one section into the other section.
In carrying out the above objects and other objects of the present invention, a method for generating a laser beam having a desired output mode is provided. The method includes providing a core having a pump input surface and at least one output surface. The core serves as a waveguide in at least one direction. The core is pumped at the pump input surface with pumping radiation at a pumping wavelength so that an output laser beam is emitted at the at least one output surface at an output wavelength. The method includes the step of separating the functions of pump-light confinement and output mode control to different sections along the length of the waveguide.
The core may have a laser input surface and wherein the method further comprises transmitting a source laser beam into the core at the laser input surface wherein the source laser beam is amplified within the core and wherein the output beam is an amplified source laser beam.
The invention provides a structure for the gain medium in a diode-pumped solid-state laser or amplifier in which the gain medium acts as a waveguide in at least one direction and in which there are separate sections along the length of the waveguide for pump light confinement and output mode control. When implemented as a side-pumped planar waveguide laser, this invention offers many advantageous features including simple power scaling, high beam quality at high power levels, high efficiency and high reliability. Compared to most other solid-state laser designs with high beam quality, the invention requires less diode pump power to achieve a desired output power and in most cases the pump diodes do not require beam-conditioning optics.
When implemented as an amplifier, the proposed invention offers higher gain than bulk rod, slab, or disk amplifiers for a given pump power due to the reduced cross-sectional area of the core. The invention also offers reduced ASE compared to a bulk rod, slab, or disk amplifier of equivalent gain. The invention also permits higher pulse energies without damage than fiber amplifiers due to the larger cross-sectional area of the core.
The emitters on high power laser diodes have typical dimensions of 50 microns to 500 microns in the xe2x80x9cslow axisxe2x80x9d and about 1 micron in the xe2x80x9cfast axis.xe2x80x9d The beam divergence is about 10xc2x0 FWHM in the slow axis. In the fast axis, the light emitted by diode lasers is highly divergent with a numerical aperture, NA, of about 0.5 (NA=sin("THgr"/2) where "THgr" is the full angle beam divergence). If laser diodes are butt-coupled to the side of the core in a planar waveguide structure with their fast axis in the guided direction, a waveguide NA greater than 0.5 is required to confine the pump light to the core. Ideally, however, the laser output from the waveguide should have a much lower NA in order to produce a good beam quality output mode from a waveguide core of a reasonable thickness. For example, a waveguide with a NA of 0.5 would require an extremely thin core thickness of about 1 micron in order to guarantee single mode output, but a waveguide with a NA of 0.02 could produce single mode output from a core thickness in the range of 20 to 50 microns. In order to minimize alignment tolerances when butt-coupling the pump diodes, it is desirable to have a waveguide core much thicker than the typical 1 micron emitter height of the pump diodes. A thicker core also simplifies fabrication. The desire to have a thick core with a high NA for efficient pump light capture and a lower NA for output mode control is addressed by this invention.
The proposed invention separates the functions of pump light containment and output mode control by moving them to different sections along the length of the waveguide. Mode control does not need to occur over the entire length of the waveguide in order to get a low NA output beam. Likewise, there is no need to pump the entire length of the waveguide. It is therefore sensible to optimize a portion of the length of the waveguide to have the desired mode control properties (a low NA) and to optimize another section of the waveguide to have maximum pump light containment (a high NA).
This can be achieved by leaving a portion of the waveguide core unclad or by depositing appropriate coatings on different sections of the core or by contacting/bonding materials with different refractive indices to different sections of the core or by a combination of these approaches. There may be other techniques as well, possibly applying a grating to a portion of the length of the waveguide for mode control.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.